1. Field
The following description relates to an optical repeater which has an increased transmission range, and more particularly, to a burst mode optical repeater.
2. Description of the Related Art
Recently, there has been increasing demand for an optical network that can transmit tens to hundreds of megabytes of multimedia content per second to subscribers at a low cost. One possible solution to meet this demand is a time division multiplexing access-passive optical network (TDMA-PON). A TDMA-PON can provide an ultra high-speed multimedia service to each optical network unit (ONU) by using an optic fiber.
A TDMA-PON consists of an optical line terminal (OLT) installed at a central office (CO) and a plurality of ONUs connected to the OLT by an optical fiber through a 1:N optical distribution node (ODN) interposed between the OLT and the ONUs. A TDMA-PON can provide a high-capacity multimedia service efficiently and inexpensively.
FIG. 1 is a block diagram illustrating a conventional time division multiplexing access-passive optical network (TDMA-PON). Referring to FIG. 1, the conventional TDNA-PON consists of an OLT which is located at a central office, a plurality of splitters which split downstream optical signals and multiplex upstream optical signals, and a plurality of ONUs which are connected to the splitters in a tree structure. In the drawing, the splitters, which are remote nodes (RNs), function as ODNs.
An optical line between the OLT and an RN is called a feeder fiber, and an optical line between an RN and an ONU is called a subscriber line.
Downstream optical signals generated by the OLT at the central office are distributed by the splitters and received by the ONUs. On the other hand, upstream optical signals generated by the ONUs are time-division multiplexed by the splitters and transmitted accordingly to the to OLT.
Here, an upstream optical signal transmitted from an ONU must not collide with an upstream optical signal transmitted from another ONU in the feeder fiber connected to the OLT.
To this end, each ONU must transmit an upstream optical signal in a time slot allocated by the OLT. In general, a continuous optical transmission generates some optical noise even is when there is no data signal. Such optical noise, which is contained in upstream optical signals transmitted from each ONU, interferes in the feeder fiber. As a result, the OLT cannot receive the upstream optical signals properly.
Therefore, each ONU must completely stop outputting an optical signal in time slots other than a time slot allocated itself in order not to generate optical noise. This transmission method is called a burst mode transmission method.
An optical repeater is located on an optical line between the OLT and the ONUs and is used to allow the transmission range between the OLT and the ONUs, and/or the number of subscribers that can be supported by a corresponding network, to be increased. That is, an optical repeater amplifies or relays upstream and downstream optical signals attenuated by transmission loss or splitter loss, thereby increasing transmission range or the number of subscribers.
Recently, the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) standardized optical repeaters by publishing G.984.6 “Gigabit-capable Passive Optical Networks (GPON): Reach Extension (ex G.984.re-GPON Optical Reach Extension)” in order to extend transmission range of a GPON.
Optical repeaters proposed in the above standard include an upstream/downstream optical repeater using an optical amplifier and an optical-electrical-optical (OEO) repeater. The proposed optical repeaters, however, are installed only between an OLT and splitters, and fail to consider the burst mode transmission method.
An optical repeater must be installed not only between an OLT and splitters but also between the splitters and ONUs.
For this reason, the present inventor has researched an optical repeater that can be installed anywhere between an OLT and ONUs in a PON, and that can receive optical signals, is which are transmitted from the ONUs to a central office using a TDMA method, and then relay the received optical signals using an optical-electrical-optical (OEO) method.